What about my plans for the summer vacation? First, I will go to Shanghai with my sister Kate. There are many interesting places in Shanghai, and we can also taste the delicious food there. After that, I will spend some time reading books. I will play tennis with my friends, too.—— Peter

    I made some plans for my summer vacation. First, I will help more at home. My parents are always busy, so I'll help them with the housework and let them have more time to relax. Second, I will practice the violin every day. Third, I think the vacation is a good time for me to do some sports, such as running and swimming. I am sure my summer vacation will be wonderful.——Tom

    After about four months' study, the summer vacation is coming. Here are my plans. First, I will leave enough time for my new lessons. I will also read some great books. Second, I will go to Xi'an and stay there for two weeks. I think the trip will be wonderful. Then, I will come back and enjoy myself with my friends. We will go swimming, shopping, etc.——Amy

在△ABC中，已知D是BC上的点，且CD=2BD．设$\overrightarrow{AB}$=$\overrightarrow{a}$，$\overrightarrow{AC}$=$\overrightarrow{b}$，则$\overrightarrow{AD}$=____ （用a，b表示）

人类最终证实地球的真实形状是（　　）